1. Field of the Invention
This invention relates to compact antennas capable of radiating different signals in the same or different directions, said signals having pure linear orthogonal polarizations.
2. Description of the Prior Art
The launching of satellites imposes strict requirements concerning size, weight, and resistance to acceleration forces of the payload. The antennas of the prior art which radiate in a wide coverage zone are generally very bulky and consequently unsuitable for use on satellites
One prior art solution to this problem is the use of separate reflectors whose respective coverage areas are juxtaposed so as to radiate in a wide zone. But this solution almost automatically implies a large-size launch configuration of the satellite.
Another prior art solution to the problem is the use of only one main reflector, and polarization filters in the region of the primary feeds. For instance, one feed is placed near the focus of the reflector, whereas the other is placed near the image of this focus, in a subreflector which filters the appropriate polarization (see FIG. 1). However, in the case of wide coverage areas, the advantage of having only one main reflector is offset by the fact that a large filtering subreflector is required. In addition, and this is the main disadvantage of this system, cross polarization induced by the main reflector is not filtered, which seriously limits the performance of such a system.
A third solution is to use an antenna system having a double grid main reflector. In this case, the main reflector comprises two orthogonal grids, offset from each other in their plane of symmetry, and based on the same mother paraboloid (see FIG. 2). This enables the same mould to be used for producing the grids. The system further comprises separate primary feeds, with horizontal and vertical polarization (Sh and Sv), which are set at different offset angles according to their associated grid reflector. However, this double grid reflector system has the disadvantage that the two reflecting surfaces are different portions of the same paraboloid, so that the mould must be considerably larger than each reflector. On the other hand, the double grid reflector does indeed provide a large coverage zone, but the offset angle of one of the reflectors may have to be large in order to achieve full coverage of that zone. For instance, the use of such an antenna to cover Europe, and more particularly a zone situated between Ireland and Turkey on one hand, and Finland and Algeria on the other, would require the setting of two reflectors on the East- and West-oriented sides of a satellite. But the angular width of Europe would impose a very large offset angle, which results in considerable aberration. Moreover, if the coverage for each type of polarization is identical, the difference between the offset angles of the two reflectors involves the use of completely different feeds for each polarization type.